Stents are devices that are inserted into a vessel or passage to keep the lumen open and prevent closure due to a stricture, external compression, or internal obstruction. In particular, stents are commonly used to keep blood vessels open in the coronary arteries and they are frequently inserted into the ureters to maintain drainage from the kidneys, the bile duct for pancreatic cancer or cholangiocarcinoma or the esophagus for strictures or cancer. Vascular as well as not vascular stenting has evolved significantly; unfortunately there remain significant limitations with respect to the technology for producing stents suitable to various portions of a patient's anatomy.
Historically, in order to provide a stent with varying characteristics, the stent had to be manufactured from multiple materials, at least one for each characteristic desired. As a result, many of these stents are woven from two or more metals having differing shape-memories for example. Unfortunately, braided stents are vulnerable to premature obsolescence. Moreover, providing multiple material types in a single stent may lead to inconsistent characteristics along the surface area of the stent. This is particularly undesirable when the stent is to be placed in vascular or nonvascular lumens that have been occluded for one reason or another. The stent needs to be stiffer in some regions while more flexible in others.
Additionally, medical device companies have identified the need to cover stents at least partially to prevent the epithelialization of the scaffolding. Most covered stents however have an elastomeric cover that is subject to bunching particularly about stenotic tissue. This can lead to additional tissue granulation. Additionally, the stents are dip coated which can lead to uneven coating as well as inconsistency in stent performance from batch to batch.
Additionally the ends of the stent tend to be exposed in order to encourage granulation tissue formation, which helps to anchor the stent in place. With metal stents, the direct metal to tissue contact accelerates tissue granulation and galvanic current generation is also an undesirable byproduct. Such direct current can have indirect effects on tissue granulation and direct effects on fluid flow dynamics.
Moreover, since many medical device companies have chosen to use poorly adapted cardiovascular stents for Pulmonary, GI and Peripheral Vascular indications, many of the anatomical differences in the lumens are not accounted for in stent design. For example, the pulsation of the cardiovascular lumen and the concomitant radial force requirements of a cardiovascular stent differ substantially from that of a tightly constricted lumen such as the trachea during repeated coughing. When a stent developed for the former is indicated for the latter, the stent tends to fail under the extreme conditions and lose its elasticity and therefore its ability of ensure airway patency. Non-vascular lumens also tend to have ciliated epithelia so as to facilitate clearance of fluids and particulates. As a general principal, coated stents were not specifically designed for ciliated lumen in that the external coating damages the cilia and prevents the body's natural clearing function. Moreover, the coating itself is usually made of a predominately hydrophilic polymer, which can lead to mucous formation and/or fluid stagnation. Stagnation of fluids or material passing through the lumen can lead to additional complications such as in stent restenosis or bacterial infections.
Therefore, there remains an existing need for a therapeutic stent that can have varying characteristics along its surface area while being stamped, not branded, from a single base material. Moreover, there is a need for such a therapeutic stent where the relative hardness, softness, flexibility, stiffness and radial force can be modified as a function of geometric considerations rather than material considerations. In particular, there is a need for a stent that is divided into zones so as to allow the stent to have predetermined characteristics in one zone and could conceivably have drastically different characteristics in an adjacent zone so as to allow for stents that can be tailored to anatomical lumens in general and the particular lumen topography of a specific patient in particular. An additional need exists for a method of differentially modifying the location, compliance, and density of the cover to achieve desired behavior. In particular, there is a need for a covered stent that is preferably covered internally such that the outer scaffolding surface of the stent is raised from the outer surface of the coating. To this end, cilia function is only partially limited and mucociliary clearance is not significantly affected. A need also remains for a coating that itself has anti-adherent properties or is complexed with an anti-adherent such that bacteria, fungi or other microbials cannot colonize the cover in particular and the stent generally. There also remains a need for a cover for the proximal and distal ends of the stent that prevent epithelialization and granulation tissue formation while achieving the benefits of traditional uncovered stents.